Central hubs for flexible magnetic data discs formed of magnetically soft polyacetal compositions

ABSTRACT

Flexible sheet-like magnetic discs (so-called &#34;floppy discs&#34;) are provided with a central hub formed of a magnetically soft polyacetal composition. The polyacetal composition necessarily includes a polyacetal resin and elemental iron particles homogeneously dispersed throughout the polyacetal resin in an amount sufficient to impart the desired soft magnetic properties to the inherently nonmagnetic polyacetal base resin. The polyacetal compositions of this invention are injection-moldable so that central hubs for flexible magnetic discs can be rapidly and economically produced.

FIELD OF INVENTION

The present invention relates generally to injection moldablemagnetically soft polyacetal compositions (to be defined below) whichare especially useful to form the central hubs of flexible sheet-likemagnetic media (e.g., so-called "floppy" data discs), and to the moldedhubs formed of such polyacetal compositions.

BACKGROUND OF THE INVENTION

Flexible sheet-like magnetic discs having a nominal standardized size ofabout 3.5 inches are widely used in a variety of data storage/retrievalsystems. For example, cassettes which include flexible magnetic discsare used in conjunction with personal computers so as to load dataand/or programs into the central processing unit of the computer, aswell as to store data in an off-site location in a more convenientmanner.

When placed into service in a magnetic recording/reproducing apparatus,the magnetic disc (which is accommodated for rotational movement withinthe interior of the cassette case) is caused to spin in the desireddirection and at the desired rotational velocity relative to a magneticread/write head by means of a motor-driven spindle coacting with anaperture in the central hub of the magnetic disc. The spindle, moreover,serves to center the magnetic disc relative to the magnetic read/writehead so that accurate placement and retrieval of data onto and from thedisc will ensue.

The recording/reproducing apparatus will also usually include anelectromagnet (usually provided integrally as part of a drive carriagewhich also includes the motor-driven spindle) which attracts the centralhub of the data disc when in service so as to ensure positive contactwith the spindle. The central hub of conventional flexible data discsmust therefore necessarily be formed of a material which exhibitsferromagnetic properties--i.e., behaves ferromagnetically when exposedto a magnetizing force. Permanent magnetic material (i.e., magnetically"hard" material) would, however, detrimentally affect the magnetic datastorage functions of the magnetic media associated with the data disc.For this reason, the data disc core is conventionally formed of amagnetically "soft" stainless steel.

Use of stainless steel as the central hub of a data disc, however,presents its own problems. For example, the stainless steel hub and themagnetic media (typically a circular flexible sheet of polyester coatedwith a magnetic film) exhibit different thermal expansion properties.There exists the possibility, therefore, that the adhesive bondingbetween the stainless steel hub and magnetic media may become loose dueto repeated thermal expansion/contraction cycles thereby rendering thedata disc unusable.

In addition, the stainless steel stock must be subjected to a number ofmetal-forming operations in order to achieve the necessary geometry anddimensional attributes required of a data disc hub. Exposure to repeatedmetal-forming operations raises the likelihood that a relatively largepercentage of stainless steel central hubs will be rejected by qualitycontrol standards. That is, since each fabrication step carries with itthe risk that the hub will not be formed to design standards for thatparticular metal-forming operation, an increase in the number offabrication steps should likewise result in an increase in the number ofrejected parts.

Recently, it has been proposed in U.S. Pat. No. 4,941,066 to Swinburneet al. to form a central data disc hub by incorporating a flat stainlesssteel insert with a molded plastics core. The formation of the metalinsert by stamping from flat metal is said to be much easier as comparedto forming a conventional hub from stainless steel. The Swinburne et al.'066 patent also mentions an alternative to the core/insert arrangementfor their central hub. Specifically, at column 4, lines 35-43, Swinburneet al. mention the possibility of forming the center core from amagnetized plastic, in which case the flat metal insert is not needed.No suggestion is made, however, of the manner in which plasticsgenerally may be magnetized. Nor is there any suggestion that magnetizedpolyacetal could be employed.

SUMMARY OF THE INVENTION

According to the present invention, magnetically soft (to be definedbelow) polyacetal compositions are provided which are especially usefulto form molded central hubs for magnetic data discs. The polyacetalcompositions of this invention are rendered magnetically soft bymelt-blending particles of elemental iron with a polyacetal base resinin amounts which impart the desired magnetic properties to thecompositions. Preferably the iron particles will be present in thecompositions of this invention in amounts in excess of about 40% byweight, and usually in amounts between about 40 to about 70% by weight(based on the total composition weight). Most preferably, the elementaliron particles will be present in the polyacetal compositions of thisinvention in an amount of about 50% by weight (based upon the totalcomposition weight).

The elemental iron particles employed in the compositions of thisinvention may be of any desired geometric shape, provided the averageparticle size is between 40 to 80 mesh.

The addition of ferromagnetic materials to polyacetal is known to causedepolymerization. Surprisingly, however, it has been found that, withthe addition of elemental iron particles, the magnetically softpolyacetal compositions of the present invention may be stabilizedagainst depolymerization using conventional stabilizers in amountsconventionally employed to stabilize acetal polymers generally (i.e.,acetal polymers which do not exhibit magnetic properties).

As briefly mentioned above, the elemental iron particles aremelt-blended with the polyacetal base resin to make the compositions ofthe present invention. In this regard, conventional screw extruders(either single or double screw types) can be employed to ensure adequateand homogeneous blending of the particulate elemental iron with thepolyacetal base resin. The melt-blended composition is then preferablyextruded into strands, allowed to solidify and then chopped into pelletssuitable for use by injection molding machinery. The central hub maythus be injection-molded using these pellets as the feed material to theinjection molding machine.

Because of the significant "loading" (in terms of weight percentage) ofthe elemental iron particles in the polyacetal base resin in order toobtain the desired soft magnetic properties, the compositions of thisinvention exhibit poor melt strength. As a result, it is important thatthe extruded strands of magnetically soft polyacetal composition bephysically supported, for example, by a conveyor or like means.

Because of the relatively low melt strengths that are attributable tothe compositions of this invention, it is also important that theextruded strands be solidified as quickly as possible after beingdischarged from the extruder in which the components are blended. Inthis connection, the extruded strands are preferably passed immediatelyinto a water quench bath. However, because the compositions of thisinvention contain elemental iron, care must be taken to minimizeexposure of the composition to aqueous environments to thereby minimizeoxidation of the elemental iron particles and the attendantdiscoloration of the polyacetal composition that would likely ensue.Thus, after being quenched in the water bath, the solidified strands arethen passed directly to a dehumidifying oven whereby essentially allwater is removed therefrom.

Central data disc hubs formed of the magnetically soft polyacetalcompositions of this invention will exhibit magnetic properties that areat least comparable to conventional hubs formed of stainless steel,while at the same time offering physical properties (e.g., reduced wear)that are markedly superior to conventional stainless steel hubs.Moreover, since the compositions of this invention are injectionmoldable, the disadvantages associated with metal fabrication techniquesare eliminated thereby potentially contributing to lesser manufacturingcost as compared to conventional stainless steel data disc hubs. And,since the data disc hubs are formed of a plastics material (polyacetal)which more nearly matches the thermal expansion properties of thepolymeric magnetically coated film, separation between the data disc andthe hub is less likely.

Further aspects and advantages of this invention will become more clearafter careful consideration is given to the detailed description of thepreferred exemplary embodiments thereof which follows.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Reference will hereinafter be made to the accompanying drawings whereinlike reference numerals throughout the various Figures denote likestructural elements, and wherein;

FIG. 1 is a perspective view of a flexible magnetic disc cassetteaccording to the present invention;

FIG. 2 is an enlarged cross-sectional view of the magnetically softpolyacetal hub employed in the magnetic disc cassette shown in FIG. 1 astaken along line 2--2 therein; and

FIG. 3 is a diagrammatic representation of the processing steps employedto make the magnetically soft polyacetal compositions of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EXEMPLARY EMBODIMENTS

Accompanying FIG. 1 shows a flexible magnetic disc cassette 10 which isconfigured to accept industry standard nominal 3.5-inch discs. In thisregard, the cassette 10 includes a cassette case comprised of upper andlower cassette case halves 12a, 12b, respectively, joined to one anotheralong their peripheral edges, for example. The lower cassette case half12b defines an enlarged opening 12c which accommodates a drive carriage(not shown) associated with a conventional magneticrecording/reproducing apparatus. The drive carriage will also include acentering/drive pin (not shown) which coact with the drive aperture 14aand centering aperture 14b defined in the central hub 14 of the magneticdisc MD.

The cassette case will also have a movable shutter 16 which is biasedvia a spring element (not shown) in a direction whereby the shutter 16is in a position which closes each of the access windows 18a, 18bdefined in the upper and lower case halves 12a, 12b, respectively. Theshutter 16 itself defines openings 16a, 16b on each of its sides whichmay be brought into registry with a respective one of the windows 18a,18b when the shutter 16 is moved into its opened position against thebias force of the spring element (not shown).

Movement of the shutter 16 into its opened position typically happensautomatically when the cassette 10 is inserted into the input slot of amagnetic recording/reproducing apparatus. The magnetic read/write headof such recording/reproducing apparatus may thus be brought intooperative association with the magnetic disc MD by virtue of theregistry of the openings 16a, 16b with a respective one of the windows18a, 18b.

The upper cassette case half 12a is most preferably provided with anintegrally molded wear button 22 which projects outwardly from theinterior surface 12a'. The wear button 22 provides a low frictionsurface against which the terminal end of the motor-driven spindleassociated with the magnetic recording/reproducing apparatus bearsduring operation. The most preferred integrally molded wear button 22 isdescribed more completely in commonly owned U.S. patent application Ser.No. 07/650,594, filed Feb. 5, 1991, the entire content of which isexpressly incorporated hereinto by reference.

The interior surface 12a' of the upper cassette case half 12a is mostpreferably provided with a magnetic disc centering ring 24 integrallymolded with, and protruding from, the interior surface 12a' of the uppercassette case half 12a in annular relationship to the wear button 22 asshown in accompanying FIG. 2. The centering ring 24 serves as a guidefor the central hub 14 of the magnetic disc MD during use, and alsoprevents lateral slippage of the magnetic disc MD within the cassettecase 12 (which could damage the same) during periods of nonuse.

The central hub 14 according to this invention is more clearly shown inaccompanying FIG. 2. As can be seen, the hub 14 is a one-piece structurein the form of a relatively shallow inverted cup having a top wall 14c,and a cylindrical side wall 14d establishing the cross-sectionaldimension of the hub 14. The centering ring 24 formed on the interiorsurface 12a' of the upper cassette case half 12a will therefore be sizedand configured to reside closely within the interior space of the hub 14established by the cylindrical side wall 14d. The centering ring 24 willtherefore serve to prevent lateral slippage of the hub 14 within thecassette 10 thereby preventing the flexible magnetic disc MD from beingdamaged, particularly at its edges.

The side wall 14d of hub 14 also includes a unitary outwardly extendingflange 14e which serves as a support for joining an interior annularconnecting region 25 of the flexible magnetic disc MD to the hub 14. Inthis regard, since the flexible magnetic disc MD and the hub 14 are eachformed predominantly of a plastics material, they could be joineddirectly to one another via heat welding or like techniques.Alternatively, the connecting region 25 of the magnetic disc MD may bejoined adhesively to the flange 14e as is conventional practice.

Important to the present invention, the hub 14. is formed of amagnetically soft polyacetal composition. The term "magnetically soft"is intended to refer to the magnetic properties that are imparted tonormally non-magnetic polyacetal resin which are characterized by highinitial and maximum permeabilities, magnetic remanence closelyapproaching saturation, and small coercive force and hysteresis loss.The magnetically soft polyacetal hub will therefore be attracted to themagnetizing force of the drive carriage, for example, thereby ensuringpositive contact with the central hub 14, and hence reliable transfer ofrotational motion thereto.

Furthermore, by the term "magnetically soft" and like terms is meantthat the polyacetal formulations of this invention exhibit magneticproperties when placed in a field but do not become permanentlymagnetized by that magnetic field. More specifically, the formulationsof this invention exhibit a magnetic value (MSV) of at least 2.0 grams,when placed in a magnetic field.

The polyacetal base resin employed in the compositions of this inventionare high molecular weight oxymethylene polymers having repeatingoxymethylene (--CH₂ O--) units. The oxymethylene polymers that maysatisfactorily be employed according to the present invention can beeither homopolymers (i.e., comprised solely-of recurring oxymethyleneunits, exclusive of endcapping units), or copolymers (i.e., comprisedmainly of recurring oxymethylene units randomly interspersed with higheroxyalkylene (preferably oxyethylene) units, exclusive of endcappingunits). The preferred oxymethylene homopolymers may be made using thetechniques disclosed in U.S. Pat. No. 2,768,994 to MacDonald, whereasthe preferred oxymethylene copolymers may be made using the techniquesdisclosed in U.S. Pat. No. 3,027,352 to Walling (the entire content ofeach being expressly incorporated hereinto by reference).

Oxymethylene copolymers comprised mainly of recurring oxymethylene unitsinterspersed with oxyethylene units are especially preferred. The mostpreferred oxymethylene copolymers are Celcon® oxymethylene copolymerscommercially available from Hoechst Celanese Corporation, EngineeringPlastics Division, Short Hills, N.J. Most preferred is Celcon® GradeMM3.5C polyoxymethylene copolymer.

The magnetically soft polyacetal compositions of this invention willnecessarily include particulate elemental iron. Preferably, theseelemental iron particles are employed in amounts no less than about 40wt. % and no greater than about 70 wt. %, based upon the total weight ofthe composition. Typically, however, the elemental iron particles willbe present in an amount of about 50 wt. %, based on the totalcomposition weight. The elemental iron particles should have a particlesize sufficient to pass through between nos. 40 to 80 mesh screens.

As mentioned briefly above, the inclusion of iron particles is known tocause depolymerization of polyacetal. Surprisingly, however, thecompositions of this invention are stabilized against depolymerizationusing conventional stabilizers in amounts typically employed instabilizing commercial grades of polyacetal resins which do not exhibitmagnetic properties. In this connection, conventional stabilization"packages" that are typically employed in commercial grades ofpolyacetal including UV-light and/or antioxidant stabilizers may beemployed in conventional amounts in the formulations of this invention.

Furthermore, other additives conventionally employed in engineeringresins may be used satisfactorily in the compositions of this invention,provided that the soft magnetic properties imparted to the compositionsby means of the elemental iron particles are not deleteriously affected.Thus, in addition to the UV-light and/or antioxidant stabilizersmentioned above, the compositions of this invention may containinorganic and/or organic fillers, reinforcing agents, mold-releaseagents, coloring agents (e.g., dyes and/or pigments), free formaldehydescavengers, and the like.

The preferred process of making the magnetically soft polyacetalcompositions of this invention is shown schematically in accompanyingFIG. 3. As is shown, pellets P of polyacetal base resin are fed into thehopper H of screw extruder SE. As is well known, the screw of the screwextruder SE forms a melt from the polyacetal base resin as it advancestowards the discharge strand die D. Elemental iron particles FeP in thedesired quantities as described above are preferably introduced into thebarrel of the screw extruder SE at a downstream location between thehopper H and the die D. Alternatively, the elemental iron particles FePmay be introduced simultaneously with the polyacetal base resin in thehopper H. The iron particles FeP are thus thoroughly mixed with andthereby homogeneously dispersed throughout the melt of polyacetal resinprior to being discharged from the die D.

The die D forms at least one, and preferably several continuous strands(usually, but not necessarily, cylindrical in cross-section) of thepolyacetal composition. Because the polyacetal composition exiting thedie D has poor melt strength as compared to "unfilled" polyacetalcompositions (compositions not having a filler material)--i.e., sincethe compositions of this invention are comprised of a significant amountof particulate elemental iron particles--the strands S exiting the die Dmust be solidified quickly. Thus, the strands S are immediately passedthrough a water bath WB containing water at ambient temperature (e.g.,approximately 70° C.). The water in the bath WB will therefore serve torapidly quench the strands S so as to maintain strand integrity.

As noted previously, since the polyacetal composition forming strand Sincludes significant amounts of elemental iron particles FeP, theresidence time of the strand S in the water bath WB must be minimized.Otherwise, there is a risk that the iron particles will oxidize therebydiscoloring the polyacetal base resin to unacceptable levels. Therefore,the residence time of the strand S within the water bath WB isminimized. Preferably, the strand resides in the water bath for no morethan about sixty (60) seconds.

Although not absolutely required, it is preferred that the at leastpartially cooled and solidified polyacetal composition strands be passedthrough an air knife AK which directs an opposing stream of cooling aircountercurrently to the direction of passage of the strands Stherethrough. The air knife AK serves to provide final cooling andsolidification of the strands S prior to their being introduced intopelletizer PZ where the strands are chopped into pellet-sized granulesG. In addition, the air knife AK serves to blow residual water from thestrand surfaces that may remain from its residence within the water bathWB.

The granules G of the polyacetal composition according to the presentinvention may then be collected in a bin B and dried overnight (e.g.,for about 12 hours) in a conventional vacuum oven or dehumidifying ovenoperating at about 140° F. to further remove residual water therefromand to thereby decrease the risk of iron particle oxidation (andpolyacetal discoloration) over time. These dried granules of thepolyacetal composition may thereafter be shipped to customers for use intheir molding operations to produce the central hub 14 having thebeneficial attributes as described above.

The following non-limiting Examples will further illustrate the presentinvention.

EXAMPLES Example I

The magnetic strength value (MSV) for compositions according to thisinvention were obtained using a modified Instron universal testingmachine having a load range capability of between 20 to 200 grams. AllMSV's were obtained using the tensile operational mode of the Instrontesting machine at a test speed of 0.2 inches per minute.

A magnet obtained from a commercially available magnetic disc drive unit(i.e., a Chinon drive with a 1.44 megabyte hard disc) was rigidlyaffixed to the stationary jaw of the testing machine using a small rod.Test specimens were mounted upon a 5/8 inch thick wooden block(approximately 6 inches by 10 inches) using double faced masking tape.

Each specimen was initially placed in face-to-face contact with themagnet so that the testing machine could be zeroed. The specimen wasthen retracted away from the magnet a dimension of 0.20 inch. Thespecimen and magnet were then moved towards one another at a speed of0.2 inches per minute until the point at which a force in a directionopposite to the direction of movement was recorded (which occurred atapproximately 0.03 inches separation distance between the magnet and thespecimen), at which time the direction of movement was reversed. Thespecimen was then subjected to the testing procedure a second time. Atrace of load versus time was recorded for each specimen using a chartspeed of 0.1 inch per minute which produced four peaks for the duplicatetesting procedures for each specimen tested. The MSV for each specimentested thus represented the average of the four peaks on the trace. Fivespecimens for each formulation were examined with the MSV's beingaveraged for the specimens attributable to each formulation.

The magnetic strength value (MSV was examined for several formulationsaccording to the present invention varying in elemental iron particlecontent from 10 wt. % to 70 wt. % using the technique described above.As a control sample, the MSV for a standard stainless steel hub takenfrom Sony MFD-2HD discs was also obtained. The results appear in Table Ibelow.

                  TABLE I                                                         ______________________________________                                        Sample No.    Fe Content                                                                              MSV (grams)                                           ______________________________________                                        1             10 wt. %  0.6                                                   2             20 wt. %  0.9                                                   3             30 wt. %  1.5                                                   4             40 wt. %  2.5                                                   5             50 wt. %  4.0                                                   6             60 wt. %  8.6                                                   7             70 wt. %  7.6                                                   Control       N/A       17.0                                                  ______________________________________                                    

The MSV noted above represents the first peak load on the trace for eachsample which generally occurred at a separation distance ofapproximately 0.03 inches. A second peak was also recorded for SampleNos. 6 and 7 (having 60 wt. % and 70 wt. % elemental iron particles,respectively) as well as the control sample when the specimen and themagnet were essentially touching. This second peak appeared to be ananomaly and was probably due to variability of results and, as such, wasdiscounted.

EXAMPLE II

Samples Nos. 4 and 5 in Example I above were tested for efficacy in adisc drive unit. In this connection, the molded central hubs of SamplesNos. 4 and 5 were assembled with magnetic media and placed within acassette case to form a standard 3.5-inch flexible magnetic disccassette. The disc cassette was then inserted into the disc drive unitof an IBM PS2 personal computer, subjected to a read/write operationwithin the drive, and then ejected from the drive using an automatedcertifying apparatus having a robotic arm designed to insert and ejectrepeatedly a floppy disc into the drive unit. Each of the disc cassetteswas subjected to a total number of 10,000 such insertion cycles. Thecentral disc formed of the polyacetal compositions of this inventionexhibited sufficient magnetic strength to achieve satisfactoryreading/writing of data onto the magnetic media during each of the10,000 insertion cycles. In addition, neither hub showed any decrease inmagnetic strength or any surface wear at the end of the 10,000 cycles.In contrast, a conventional stainless steel hub similarly subjected to10,000 insertion cycles exhibited noticeable surface wear indicatingthat fine stainless steel particles were perhaps released within thedisc drive unit.

EXAMPLE III

An iron-loaded polyacetal composition was prepared by blending 50 wt. %of elemental iron particles having an average particle size of betweenabout 40 to about 80 mesh screen (U.S. Standard Mesh in a conventionalscrew extruder operating at average barrel temperature of about 375° F.In this connection, the iron particles used in this Example III werecommercially obtained from the Hoeganaes Corporation of Riverton, N.J.and identified as M2080 iron particles. The iron-loaded polyacetalcomposition was extruded through a die maintained at a temperature ofabout 375° F. to form strands. These extruded strands were thenimmediately introduced through a quench bath containing room temperature(70°-75° F.) water. The quench bath was three (3) feet in length so thatthe strand residence time in the quench bath was only approximately 30seconds. These quenched strands were then sent to a pellitizer where thestrands were chopped into pellets. The pellets were collected and driedovernight in a dehumidifying oven to prevent discoloration of thecomposition due to oxidation of the elemental iron.

EXAMPLE IV

The composition of EXAMPLE III was molded into two (2) flat test bars(identified as B1 and B2) approximately 1/2 inches wide×41/3 incheslong×1/8 inch thick and subjected to magnetic field testing to determinewhether the composition exhibits "soft" magnetic properties. Initialmagnetization readings at several locations along the top and bottomsurfaces for each of the test bars were obtained using a Gaussmeter(F.W. Bell Model 615). These initial magnetization readings appear inTable 2a below. The B1 test bar was then magnetized repeatedly usingmagnetizing apparatus (F.W. Bell Model 861A with model 8626A basicmagnetizer) set at a capacitor voltage of 350 volts. After eachmagnetization cycle, the B1 test bar was examined for magnetizationproperties at several locations along one of its surfaces using the sameGaussmeter as was used for the initial magnetization examination. Thelowest and highest magnetization noted after each magnetization cycleappears in Table 2b below.

                  TABLE 2a                                                        ______________________________________                                        Initial Magnetization                                                         ______________________________________                                                Gauss Readings - Top Surface                                          Test Bar  No. 1         No. 2   No. 3                                         ______________________________________                                        B1        .000          +.001   -.003                                         B2        .000          -.002    .000                                         ______________________________________                                        Gauss Readings - Bottom Surface                                               Test Bar                                                                              No. 1    No. 2    No. 3  No. 4  No. 5                                 ______________________________________                                        B1      +.002    +.001    -.002  -.001  -.002                                 B2       .000     .000     .000   .000  -.003                                 ______________________________________                                    

                  TABLE 2b                                                        ______________________________________                                        Magnetization After Magnetization Cycles                                      Gauss Readings - Test Bar B2 Surface Locations                                Cycle   No. 1    No. 2    No. 3  No. 4  No. 5                                 ______________________________________                                        1       -.002    -.003    +.001  -.004  +.003                                 2       +.001    -.003    +.004  -.005  +.004                                 3        .000    -.004    +.004  -.002  +.001                                 4       -.001    -.004    +.001   .000   .000                                 5       -.002     .000    +.004  -.005  +.003                                 6        .000    +.002    +.003  -.004  +.003                                 7        .000    +.001    +.002  -.003  +.002                                 ______________________________________                                    

The above data show that the polyacetal compositions of this inventionexhibit "soft" magnetic properties since repeated exposure to magneticfields does not cause any meaningful increase in the magnetizationproperties. In addition, the data after repeated magnetization arecomparable to the initial magnetization properties which furtherindicates that no permanent magnetization of the polyacetal compositionsof this invention ensue.

Although the present invention has been described in connection withwhat is presently considered to be the most practical and preferredembodiment, it is to be understood that the invention is not to belimited to the disclosed embodiment, but on the contrary, is intended tocover various modifications and equivalent arrangements included withinthe spirit and scope of the appended claims.

What is claimed is:
 1. A central hub for a flexible magnetic disc whichconsists essentially of a magnetically soft polyacetal resincomposition, wherein said composition comprises a polyacetal base resin,and elemental iron particles homogeneously dispersed throughout saidpolyacetal base resin in an amount of at least 40% by weight, based onthe total weight of said polyacetal resin composition, sufficient toimpart magnetically soft properties to said polyacetal base resin suchthat said central hub exhibits a magnetic strength value of at least 2.0grams when placed in a magnetic field but is not permanently magnetizedby said magnetic field.
 2. A central hub as in claim 1, wherein saidpolyacetal resin is selected from oxymethylene homopolymers andoxymethylene copolymers.
 3. A central hub as in claim 1, wherein saidpolyacetal resin is a polyoxymethylene copolymer consisting essentiallyof repeating oxymethylene units interspersed randomly with higheroxyalkylene units.
 4. A central hub as in claim 3, wherein said higheroxyalkylene units are oxymethylene units.
 5. A central hub as in claim1, wherein said elemental iron particles are present in an amountbetween about 40% to about 70% by weight, based on the total weight ofsaid polyacetal resin composition.
 6. A central hub as in claim 1,wherein said elemental iron particles have an average particle size ofbetween 40 to 80 mesh.
 7. A magnetic data disc cassette comprising:acassette case sized and configured to be accepted within a disc driveunit equipped with a spindle magnet; and a data disc housed by saidcassette case for rotational movement therewithin, said data discincluding (i) a central hub, and (ii) a flexible sheet of magnetic mediafixedly attached to a peripheral region of said central hub, whereinsaid central hub is a one-piece injection molded structure whichconsists essentially of a magnetically soft polyacetal resincomposition, wherein said composition comprises a polyacetal base resin,and elemental iron particles dispersed throughout said polyacetal baseresin in an amount of at least 40% by weight, based on the weight ofsaid polyacetal resin composition, sufficient to impart magneticallysoft properties to said polyacetal base resin such that said central hubexhibits a magnetic strength value of at least 2.0 grams when placed ina magnetic field but is not permanently magnetized by said magneticfield.
 8. A data disc cassette as in claim 7, wherein said polyacetalresin is selected from oxymethylene homopolymers and oxymethylenecopolymers.
 9. A data disc cassette as in claim 7, wherein saidpolyacetal resin is a polyoxymethylene copolymer consisting essentiallyof repeating oxymethylene units interspersed randomly with higheroxyalkylene units.
 10. A data disc cassette as in claim 9, wherein saidhigher oxyalkylene units are oxyethylene units.
 11. A data disc cassetteas in claim 7, wherein said elemental iron particles are present in anamount between about 40% to about 70% by weight, based on the totalweight of said polyacetal resin composition.
 12. A data disc cassette asin claim 11, wherein said elemental iron particles have an averageparticle size of between 40 to 80 mesh.